High Volume Hot Water System Design Help

I'm new to this forum but have been lurking for weeks looking for info on a new hot water system.

We have a full (to studs) remodel in progress on our house and have plans for a large shower in the master. I want to make sure we have the correct water heating setup to ensure we can enjoy the shower we've paid so much for. I'm not looking for info on which setup (i.e. tankless vs standard vs hybrid) is more efficient or will have a faster payback...just what will produce the results we desire. I also want to ensure we don't get the cold water sandwich, or problems with pressure balancing valves, hot water to the high efficiency appliances, etc.

House has 4 total bathrooms. System will feed 2 standard showers, a large soaking tub, the large master shower, high efficiency washer, water miser dishwasher, two kitchen sinks, 4 bathroom lavs. Heater location is in basement.

My main area of concern is the master shower. It is a 2 head shower, each head with 3 body sprays, plus one large rain shower head overhead. I know it sounds wasteful and it will probably only be used 1 out of every 90 days at full capacity, but I'd like to be able to run the shower as it's designed (i.e. all heads running) without worrying about running out of hot water. Otherwise why spend the money??

I'd love any suggestions on system setup from all the experts here. My current plumber (who doesn't seem to have a ton of system design experience) has suggested two State Commercial 130 Gal, 500,000 BTU Water Heaters Water Heaters in parallel at a whopping 12K a piece. 24K in 2 water heaters seems a little overkill for one shower but I'll use you guys as a reality check. I'm open to tankless, tankless with tank, tankless with recirc, standard, hybrid...all of em as long as I get realiability and it works. I can run new lines as necessary. I'm also open to different heaters feeding different loads if necessary.

So, theoretical maxed out you're looking at 14gpm (which IS a real energy pig, no doubt!).

A 70F rise on 14gpm equals about 490,000BTU/hr.

A single two 80% 500K burner is putting out 400K, so putting two of those monsters in parallel is a wee bit overkill.

If you have 4-8 feet of vertical drain downstream of this font of indulgence, you can get the rest of the heat (and then some from 1 (or two paralleled-up to be able to achieve the flow) drainwater heat exchangers (DWHX) for under a grand per, and the thing would be able to run full blast 24/365 with just one of the hot water heaters. Of the current market offerings, the Renewability PowerPipe series has the lowest pressure drop across the DWHX with flow, and a pair of 4" x 48" or 3" x 60" (or longer, on either) plumbed such that it feeds both the cold side of the shower and the cold input to the water heater should get you there. At 2.5gpm per they'd be returning more than 50% of the heat, and at 7gpm per (14 total) they'd still be delivering over 40%, so with 400K of burner and (0.4x 490K=) ~200K of DWHX you'd have BTU-capacity to spare in showering mode.

It's possible to parallel them up in arbitrarily wide gangs, and getting both the drain an potable flows to split evenly takes a careful bit of plumbing, but if it's even close to evenly-split you'll get there.

Best part about it is that the 200KBTU/hr delivered by the DWHX uses no fuel, and the total energy use is roughly in half when showering, which adds up fast with a gusher like that.

The fatter and longer, the higher the BTU-efficiency, but the higher the pressure drop (due to the longer length of potable wrap). Figure out what fits, and figure on at least two. It's easier to balance flows with 4 than with 3, but you can probably get sufficient flow out of 2. Call the manufacturer if you can't figure out how to parallel them, but the above picture of the 4-fer tells the story pretty well.

Wow, look at all that copper. Better mot let the crack heads see that or it will be in the scrap yard by morning. Does look like a good idea to reclaim all that heat going down the drain. I also think 2 120gal tanks is over kill but have no idea how to size it properly

Sizing for a mega-BTU gas service just for hot water heating would also be an issue, but it's done all the time in huge houses with pool heaters, etc.

All that copper is still a heluva lot cheaper than than $12 grand, installed, even for 4. And 2 will probably do just fine on flow without having to crank up the water pressure. Just one wouldn't cut it at 14 gpm though.

Another way to get there is to gang tankless heaters together to achieve almost any arbitrary flow number. To get there it could take three 199KBTU/hr tankless water heaters with an external contoller, and it's likely that three Rinnai or Noritz or Takagis ganged with an external controllers would come in under $12K for the package.

Alternatively, a pair of ganged 250BTU/hr commercial tankless burners plus a pair of drainwater heat exchangers would do it, probably for less cash up front than three 199KBTU/hr residential tankless units, and would use half the fuel, and take less space than the 130 gallon tank solution. (I'd probably opt for this one if it were my place)

Thanks for all the info. I've started researching the tankless commercials too...seems like I could get to the 500,000 BTU's needed even without the DWHX. I need to verify we have the vertical space to put a DWHX though, because I do like the idea. In your calc above are you assuming 14 gpm because of the hot/cold mix ratio? Just trying to undertand everything.

Seems like two Noritz or Rinnai Commercials would get me the heat needed for this thing, and at a lot better price than 24K before installation, but what about the washer and dishwasher? Should I think about a 20 gal Electric after the tankless combo to ensure those appliances work correctly?

Another builder suggested I put the washer and dishwasher on their own small tank heater.

I'll verify for sure we have enough flow for all the heads, but I'm fairly sure we do. The neighbors on each side have similar mostrosities to keep their wives happy in their homes too and they have not upgraded their supply lines. I'm trying to get into their basements to see their setups too. My hope is they don't have the same setup as my plumber recommended -- not sure the wife's happiness is worth 24-30K just to heat the water.

The only other thing I'm worried about with the tankless is the pressure balance issues. Are these legit or just issues from earlier installations that can/have be corrected?

The house a few doors down did a big remodel. After the walls were opened up and the plumbing reworked the contractor arrived one morning to find all his hard work removed by the local meth head. Those things would look like liquid gold to these guys.

Anyway...just a sidebar. Don't want to detract from my original goal of information gathering for this.

Thanks for all the info. I've started researching the tankless commercials too...seems like I could get to the 500,000 BTU's needed even without the DWHX. I need to verify we have the vertical space to put a DWHX though, because I do like the idea. In your calc above are you assuming 14 gpm because of the hot/cold mix ratio? Just trying to undertand everything.

Seems like two Noritz or Rinnai Commercials would get me the heat needed for this thing, and at a lot better price than 24K before installation, but what about the washer and dishwasher? Should I think about a 20 gal Electric after the tankless combo to ensure those appliances work correctly?

Another builder suggested I put the washer and dishwasher on their own small tank heater.

I'll verify for sure we have enough flow for all the heads, but I'm fairly sure we do. The neighbors on each side have similar mostrosities to keep their wives happy in their homes too and they have not upgraded their supply lines. I'm trying to get into their basements to see their setups too. My hope is they don't have the same setup as my plumber recommended -- not sure the wife's happiness is worth 24-30K just to heat the water.

The only other thing I'm worried about with the tankless is the pressure balance issues. Are these legit or just issues from earlier installations that can/have be corrected?

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Yes, the 14gpm is the full flow of the shower, all heads running, which is the relevant number. Only if you set the output temp of the HW heater to a showering temp (~104-107F) would all of that flow be going through the hot water heater, but the BTUs required to raise the incoming water to showering temp is the same, whether 100% of the flow is through the water heater set to 105F or if it's mixing with cold water when set to a higher temp. For most homes you'd want to set the temp to at least 110F for filling tubs, and 115F-125F would be reasonable for a tankless. (For tanks you want it even higher to mitigate pathogen growth.)

With 500,000BTU of 90% efficiency burner you'd still be at only 450BTU/hr to serve a 490BTU/hr estimated peak load. A typical condensing heater hit's 95%+ efficiency only at the lower third of it's firing rate. Even it ran at 95% steady efficiency at full fire you'd still be only looking at 475BTU/hr. The drainwater heat recovery is still critical to avoid having to go to a third tankless. Don't count on anything more than 450K out of a 500K commercial tankless burner even if the documentation indicates "98% efficiency".

The pressure balance issue is a function of the anti-scald mechanisms on shower mixers, but most can tolerate at tankless. With a drainwater heat exchanger in the system the flow & pressure-drop through the tankless is lower too, since it takes less 120F water to mix to 150F when mixing in ~75F tepid water out of the DWHX than when mixing in ~40F cold water from the street, which further mitigates those issues. Setting the output temp of the tankless higher and mixing it down to ~120F at the heaters means the same shower temp can be achieved with lower flow through the tankless too- there's always a way to get there if it's a problem, and it's usually not a problem.